Analysis of the heart rhythm is made based upon electrogram (EGM) signals, collected using electrodes mounted on endocardial leads, implanted in the myocardium. From the EGM, one can measure the atrial and/or ventricular depolarization potential. These signals are then analyzed by the implantable device (IMD), which, when appropriate, will deliver to the patient an appropriate therapy. The therapy delivered may be, for example, in the form of low energy pulses (anti-bradycardia pacing or ventricles resynchronization pacing) or cardioversion or defibrillation shocks.
However, the rhythm analysis, and therefore the decision to deliver or not a therapy, may be altered by artifacts collected by the endocardial lead.
These artifacts can have various origins. A first series of artifacts corresponds to such situations when the device not only detects the event as such, i.e, the depolarization wave of the considered cavity, but also a disturbance that is associated with that same event and considered, erroneously, as another event occurring after said first event: e.g., a late depolarization wave, cross-talk between the two cavities, etc.
Another series of artifacts are those to which the present invention is directed, namely artifacts of extrinsic noise not related to the myocardium depolarization. This noise can have various origins: notably, the myopotentials associated with muscular contractions, as well as electromagnetic interference (EMI) coming from different electronic equipment, such as surveillance devices, current daily-life devices, electro-surgical devices, communication systems, etc.
Such noise, if present with more or less regularity, can then be detected by the IMD as a myocardium depolarization, with a risk to generate inappropriate therapies, for instance by erroneously inhibiting the antibradycardia pacing therapy or resynchronization therapy, or, conversely, by erroneously delivering inappropriate shocks.
Various techniques have been proposed to reduce the influence of such extrinsic noises, notably the application of analog or digital filtering, the implementation of refractory periods, the automatic adaptation of sensing amplifier sensitivity, or the automatic gain control of these amplifiers.
However, the use of such techniques is always detrimental towards a good sensing.
In particular, in order to ensure the sensing of ventricular fibrillation (VF) with a low signal level, it is necessary to seek a maximal sensing sensitivity, so as to minimize the risk not to sense some events that should have been sensed. Indeed, the amplitude of VF signals may have a variable level comprised between the level of noise signals likely to be sensed by the IMD, and that of the signals of sinus complexes. If ventricular fibrillation has to be sensed, the sensing of a potential noise therefore cannot be avoided. If, moreover, a steady noise is present, with a patient presenting a normal cardiac sinus rhythm, such noise may be confused with depolarizations. That situation may distort the evaluation of the average rhythm by the IMD, such rhythm being evaluated at a level much higher than reality, with a correlative risk for applying an undesirable antitachycardia therapy (false positive). Conversely, if the IMD is set with too low a sensitivity, that is with a sensing threshold too high, the true episodes of ventricular fibrillation may not be sensed (false negative), with consequences that are much more severe towards the patient.